10. They're Survivors
Surviving traumatic experiences can bond people together. In the case of Kira and Sisko, that was definitely the situation. Throughout the course of Deep Space Nine, the duo stood side by side through battles, invasions and outright war. Coming out the other side of those moments forged a connection difficult to break.
But their bond as survivors goes much deeper than that.
Kira grew up as a resistance fighter during the occupation of Bajor by the Cardassians. Alongside her fellow cell members, she tried to pay her people's oppressors back for every horror visited upon them. This war made Kira who she was but it also left scars, the kind that someone like Sisko could see.
Sisko's tale of survival was much different than Kira's. His stemmed from the confrontation with the Borg at Wolf 359. Led by Locutus, the name given to the Picard when he was assimilated, Sisko's ship was one of many that were destroyed. He and his Jake barely escaped with their lives. His wife did not.
Both Kira and Sisko carried the long term pain from these events with them to Deep Space Nine. It was there that they found ways to move on, in no small part to the support they gave each other.